The present invention relates to an image forming apparatus and, more particularly, an image forming apparatus for forming an image on an image formation medium by using liquid toner.
Image forming apparatuses using liquid toner have advantages over, for example, dry type apparatuses such as electrophotographic apparatuses and electrostatic recording apparatuses. Merits of such apparatuses have come to be redeemed. For example, the main merits of these apparatuses are that high image quality can be realized because fine liquid toner of the order of submicron size can be used, economical operation can be realized because sufficient image density can be obtained with a small amount of liquid toner, and near-typeset-quality texture can be obtained.
On the other hand, conventional image forming apparatuses using liquid toner have suffered several problems, and hence have allowed the dry type techniques to dominate the market for a long period of time. One of these problems is associated with a transfer means.
The first problem in transfer is that the image quality is poor. More specifically, according to a conventional technique, as a transfer means directly transfers liquid toner adhering to an image carrier onto a paper sheet by using an electric field, transfer unevenness is caused by electric field variations corresponding to unevenness on the surface of the paper sheet. In addition, a transfer failure tends to occur due to variations in the electric characteristics of the paper sheet, environment dependency, and the like, resulting in a considerable deterioration in the image quality of the transferred image.
In order to solve such a problem, an apparatus has been proposed, which temporarily transfers an electrostatic latent image from an image carrier onto an intermediate transfer medium and then transfers it from the intermediate transfer medium onto a paper sheet. Apparatuses for transferring an electrostatic latent image from an image carrier onto an intermediate transfer medium by using en electric field and then transferring it from the intermediate transfer medium onto a paper sheet by using pressure (and heat) are disclosed in, for example, U.S. Pat. Nos. 5,148,222, 5,166,734, and 5,208,637.
In this case, since it is relatively easy to make an intermediate transfer medium by using a material having a smooth surface and exhibiting little fluctuations and variations in electric resistance, a deterioration in image quality due to transfer can be reduced. However, a transfer efficiency of 100% cannot be achieved. In addition, since electric field transfer uses electrophoresis, a large amount of solvent must be left in a liquid toner image (visible image) in a transfer process. This solvent moves to the intermediate transfer medium and evaporates due to heat, and hence a large amount of solvent vapor is generated.
Apparatuses which use pressure (and heat) for both transfer onto an intermediate transfer medium and transfer onto a paper sheet are disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 46-41679 and Jpn. Pat. Appln. KOKAI Publication No. 62-280882. In many cases, since the great part of the solvent in a liquid toner image must be evaporated almost completely before primary transfer onto an intermediate transfer medium, the solvent after development is squeezed out. As a consequence, the amount of solvent vapor generated decreases.
If, however, pressure (and heat) is used for both transfer onto an intermediate transfer medium and transfer onto a paper sheet, the releasability (surface energy) of the intermediate transfer medium and photosensitive body is difficult to adjust. According to an experiment, both transfer processes cannot be satisfactorily performed unless the surface energy of the intermediate transfer medium is set to about 30 to 35 dyne/cm and a release layer with 30 dyne/cm or less is formed on the surface of the photosensitive body. It is especially difficult to perform transfer onto paper with poor smoothness, e.g., PPC paper. If, therefore, a sufficient transfer efficiency is to be obtained, transfer onto the intermediate transfer medium becomes unstable.
As a technique of solving such a problem, a transfer scheme is available, which uses an intermediate transfer medium which is an elastic member having relatively low surface energy and has tack on its surface. In a primary transfer process, a liquid toner image on a photosensitive body is transferred onto the intermediate transfer medium by using the tack strength (slight adhesive strength) between the intermediate transfer medium and liquid toner. In a secondary transfer process, the releasability of the intermediate transfer medium is mainly used to transfer the image onto a paper sheet using pressure and heat.
In primary transfer using the above tack, the state of a liquid toner layer on an image carrier (photosensitive body) is very important. More specifically, while the liquid toner is completely dissolved, the liquid toner layer becomes a uniform film, and particles retain nothing of their original form. When the temperature of the surface of the photosensitive body is equal to or higher than the glass transition temperature (Tg) of liquid toner, in particular, the liquid toner completely becomes filmy.
Under this condition, the thinner the liquid toner layer formed on the photosensitive body is, the more difficult primary transfer is. This is because as the liquid toner layer becomes thinner, the tack of the layer as a film is lost. In fact, since the particle size of a pigment dispersed in a resin is about 0.05 to 0.2 xcexcm, when the overall thickness of the liquid toner layer becomes 0.2 xcexcm or less, the pigment without tack takes the form of pillars in the direction of thickness. As a consequence, it is difficult for the entire film to obtain an elastic effect. According to an experiment, when the thickness of the liquid toner layer becomes less than 0.2 xcexcm, the primary transfer efficiency abruptly decreases. As described above, in the conventional apparatuses, since thin liquid toner layer portions are formed, it is difficult to attain image quality at a certain level or more.
It is an object of the present invention to provide an image forming apparatus which can form a high-quality image by stably performing proper transfer even when the liquid toner layer on an image carrier is thin, and can also reduce power consumption.
According to the present invention, there is provided an image forming apparatus comprising determination means for determining whether the supplied image signal is an image signal that makes a liquid toner film have a thickness not more than a predetermined value, image processing means for performing predetermined processing for the image signal and outputting a processed image signal when the determination means determines that the supplied image signal is an image signal that makes the liquid toner film have a thickness not more than the predetermined value, electrostatic latent image forming means for forming an electrostatic latent image on the image carrier by irradiating the image carrier with a light from exposure means on the basis of the processed image signal output from the image processing means, developing means for developing the image on the image carrier by using the liquid toner in accordance with the electrostatic latent image formed on the image carrier by the electrostatic latent image forming means, and transfer means for transferring the image on the image carrier, developed by the developing means, onto the recording medium.
In the image forming apparatus of the present invention, according to the above features, whether an image signal is likely to generate a low-density image portion is checked by comparing the image signal with data such as data in a table in the image processing section. If it is determined that a low-density image portion is likely to be generated, signal processing is performed to remove this low-density image portion by, for example, amplifying the signal or combining separate pulse signals. This prevents the generation of a low-density image portion, which is generated when an image is formed by using liquid toner in the prior art, thus providing an image forming apparatus which can form a high-quality image without any loss or the like.
A method of removing a low-density image portion resulting from this liquid toner is not limited to this. For example, such a portion can also be removed by additionally applying a transparent resin liquid toner or controlling the temperatures of the image forming apparatus or intermediate transfer medium in consideration of the glass transition temperature of liquid toner.
According to these various embodiments as well, image forming apparatuses which can form high-quality images by using liquid toner can be provided.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.